This invention concerns a vertical bi-polar transistor and a procedure for its manufacture.
The collector resistance, which has a decisive influence on the high-frequency properties of bi-polar transistors, usually has a non-negligible parasitic lateral component in the conventional transistor designs.
This occurs because the conventional designs of vertical bi-polar transistors use a collector terminal that is laterally separated from the base zone of the active (inner) transistor by field oxide, and a base zone that usually completely surrounds the emitter in order to keep the base resistance low, which is also important for the high-frequency properties of the transistor. With a given minimum lateral expansion of the base zone important for achieving a minimal base resistance, which is determined by the existing design rules for the mask procedures, the minimum distance between the collector and the inner transistor is set and it complies at least with the minimum dimensions determined by the structure-building process.
The size of the parasitic lateral component of the collector resistance is influenced, besides the lateral dimensions of the field oxide zone, also by the layer resistance of the sub-collector. The sub-collector consists of a highly doped semi-conductor zone, which is separated, in vertical direction, from the base zone of the active transistor by a significantly less doped collector layer mostly produced by epitaxy. A reduction of the layer resistance under the already reached and practically used level may not be technically possible. However, on the contrary it is desirable to achieve very good high-frequency properties also with relatively easy-to-build xe2x80x9cretrogressivelyxe2x80x9d implanted collector profiles, for the manufacture of which is used neither an epitaxy layer nor a collector manufactured separately, and which, therefore, are connected with a relatively high collector resistance. Although there exist well-known solutions such as EP 227 970 B1 how to achieve useful collector resistance and good high-frequency properties with such implanted retrogressive collector profiles, these solutions have the disadvantage of a costly manufacturing process with at least two mutually overlapping polysilicon levels for the base and the emitter. This is why the desired cost-reducing simplification of the manufacturing process is not achieved or is achieved only partially with the aforementioned well-known solutions.
The task of this invention is to propose a vertical bi-polar transistor and a procedure for its manufacture, while achieving excellent high-frequency properties of the transistor with a manufacturing technology as simple as possible that requires an implanted epitaxy -free collector and only one extensively deposited polysilicon level, and is easily integrated in a conventional mainstream CMOS process without tub zones produced by epitaxy.
According to this invention, a simplification of the manufacturing technology with a simultaneous improvement of the high-frequency parameters of vertical bi-polar transistors through a reduction of parasitic lateral and vertical collector resistance components is achieved by means of a self-adjusting transistor design in connection with a special manufacturing procedure, during which a highly doped monocrystal base zone surrounding the active base is removed in the area of the collector by ion etching together with the underlying less doped zone of the collector or a part thereof. This enables a self-positioning of the deeper lying collector terminal to the base zone, which allows to reduce to a functionally required minimum the distance between the silicated collector terminal and the equally silicated monocrystal base zone by means of a spacer conventionally manufactured from an insulating material during the etching process. The elimination of the less doped collector zone avoids the necessity of the so-called collector shaft implantation, and the vertical component of the collector terminal resistance is equally reduced. Before the self-positioning, preferably joint siliconization of the base and the collector zone according to this invention, only a flat high-dose implantation is required such as the manufacture of flat S/D zones usual in modem MOS technology. Only narrow edge areas of the base and collector zones (covered with a spacer made of insulating material or with an emitter overlapping the base zone) are exempt from the siliconization. The thus produced collectorxe2x80x94which is self-positioned to the base zonexe2x80x94usually surrounds the inner transistor on three sides so that the lateral component of the collector resistance is further minimized in the sense of the task of this invention. If the bi-polar transistor manufactured according to this invention is integrated in a CMOS process, the siliconization of the base and collector terminal may be purposefully performed together with the siliconization of the S/D and gate zones.
In an especially advantageous design form of the invention, the base zone comprises a second outer sub-zone, which consists of a metal silicide on a highly doped polycrystal layer of the same material and is of the same doping type as the inner monocrystal sub-zone. This outer sub-zone is separated from the collector or from the monocrystal semiconductor substrate by a thick insulation layer, and therefore has, as compared with the first subzone, a very tiny capacity to the collector or the monocrystal semi-conductor substrate.
In another especially advantageous design form of the invention, the collector and sub-collector of npn bi-polar transistors are a part of a retrogressive doped profile manufactured by P ion implantation in a semi-conductor substrate. The doped profile can be also used as a domain for MOS transistors integrated on the same substrate. Although the subcollector of the thus manufactured bi-polar transistor has a many times higher layer resistance as compared with the conventional transistor designs with an epitaxial collector and a highly doped buried subcollector layer, the collector resistance is comparable with the resistance of standard designs with epitaxy and a buried subcollector.
The inner base and the monocrystal sub-zone of the base zone preferably consist of epitaxially deposited silicon or silicon-germanium. Designs with an implanted base zone are, however, also possible within this invention, and the invention, as described here, can also be used with conventional bi-polar substrates with a subcollector of low-impedance and an epitaxial collector to further reduce the collector resistance as compared with the known standard designs and thus e.g. to increase the cutoff frequency f1 and the maximum oscillation frequency fmax beyond the values so far achieved by the standard designs.
This design can be technologically realized in an especially advantageous manner by first producing a subcollector, a collector, a basis and an insulation layer covering them in a continuous active zone, which will later also contain the subsequent collector terminal, and then introduce emitter windows in the insulating layer. Subsequently, an emitter is deposited, which overlaps the emitter windows and preferably consists of highly doped polysilicon. Afterwards, and preferably self-adjusting to the polysilicon of the emitter, the base zone surrounding the emitter is highly doped (as compared to the inner base), and subsequently completely removed in the area of the subsequent collector terminal by means of a suitable anisotropic etching procedure, together with at least a part of the weakly doped collector. During this etching procedure, the inner base including the emitter that covers it and the subsequent base zone are protected by a coating or hard mask. A self-adjusting spacer from insulating material, which was produced on the almost vertical stage arising during the etching, prevents the creation of silicide on the wall of this stage during the ensuing salicide process with the formation of salicide on the deeper lying collector terminal and preferably also simultaneously in the base zone.
If an epitaxial base is used, at least a part of the polycrystal layer, which forms the outer sub-zone, is simultaneously deposited during the epitaxial depositing of the first subzone of the base zone. The self-positioning implantation of a highly doped semi-conductor zone under the metal silicide of the collector terminal is made possible in another advantageous procedure variant by an auxiliary mask, which is impervious to the aforementioned implantation and which is used during a reactive ion etching process applied to the base and to the weakly doped collector zone, and further made possible by a spacer produced on the etching edges after the etching of the weakly doped collector zone.
Other advantageous procedure forms are described in the corresponding sub-claims and in the design example.